DESCRIPTION: This application aims to investigate the presence and the activation mechanisms of precursor cells in lung tissue of patients with pulmonary arterial hypertension (PAH), a group of deadly diseases without curative treatment options. It is increasingly acknowledged that the cells in the angioproliferative lesions are apoptosis-resistant, and recent years have generated substantial information about the contribution of bone marrow (BM)-derived precursor cells to vascular remodeling mainly in animal models, but it is unknown whether or not BM-derived and resident precursor cells contribute to angioobliterative PAH in human patients. The current literature also lacks a detailed understanding of the mechanisms of precursor cell activation in PAH. Preliminary data in our animal model of SU5416/chronic hypoxia (SuHX) indicates that precursor cells expressing various stem and progenitor cell markers accumulate in and around angioobliterative lesions and that some of these cells may be self-renewing stem cells. In human tissue of PAH patients, we have detected a similar distribution of cells staining positive for these stem and progenitor cell markers as in the SuHX model. This proposal is an extension of our animal work to detect stem and progenitor cell marker, activation/differentiation factors by immunofluorescence staining and confocal microscopy, as well as angiogenic and precursor activation potential of human PAH lung tissue and serum by in vitro angiogenesis assay and cell culture. Although animals models are important to investigate disease mechanisms and treatment strategies, it is necessary to examine human tissue specimens to increase the translational value of our animal data.